This invention relates to digital modulators, and, more particularly to digital modulators in which the modulation of a digital signal onto a carrier occurs entirely in the digital domain.
Digital modulators are employed when it is desired to transform a digital data stream into an analog signal modulated onto a carrier or intermediate frequency signal. Digital modulators are used, for example, in cable modems, television set top boxes, Microwave Multipoint Distribution Systems, Local Multipoint Distribution Systems, Orthogonal Frequency Division Multiplexing, and Vector Orthogonal Frequency Division Multiplexing.
In a classical implementation of a digital modulator, a pair of digital data streams, perhaps derived from a single data stream, are converted into analog signals and mixed as analog signals with the outputs of a quadrature oscillator to produce in-phase and quadrature-phase signals imposed on a carrier at the frequency determined by the oscillator.
Other more recent digital modulators, known as direct digital synthesis (DDS) modulators, have provided similar digital data streams (which may be designated as the real and imaginary components of an original data stream) which are modulated onto a carrier signal entirely within the digital domain. This allows processing of the signal substantially entirely within one integrated circuit. Once the digitally modulated carrier is produced, it is converted to an analog modulated carrier by a digital to analog converter.
This latter technique has several advantages, including the introduction of less distortion due to filter dissimilarities in the in-phase and quadrature-phase analog paths, better gain balance, and better phase balance.
The oscillator of a DDS, however may introduce spurious signals and noise into the system, thereby reducing the effectiveness of the circuit. It would be desirable to retain the advantages of direct digital synthesis while at the same time reducing the spurious content of the oscillator.
The above and other advantages of the present invention are attained by providing a method and apparatus for generating digitally modulated signals in which a serial data stream of digital signals to be modulated is provided, the serial data stream being converted into real and imaginary components which are then converted into a complex polar signal representing the serial data stream. A carrier of appropriate frequency is generated by an infinite impulse response filter and the polar signal is mixed with the output of the infinite impulse response filter to provide a representation of the complex polar signal modulated at the frequency generated by the infinite impulse response filter. Subsequently the imaginary component of the resulting representation is stripped from the signal and the real component of the resulting representation is applied to a digital to analog converter to produce an analog version of the serial data stream.